As an electric motor of a type that generates a rotational force by using a difference in magnetic resistance between a stator and a rotor, a reluctance motor has been known. In the reluctance motor, the rotor is rotated by reluctance torque caused by a difference in magnetic resistance. However, the reluctance torque is smaller than torque obtained from a magnet. Therefore, compared with a motor that uses a magnet and is the same in size, the reluctance motor tends to have a smaller output torque. In recent years, what has been proposed is a magnet assistance-type reluctance motor that has the same basic configuration as the reluctance motor, with a magnet being disposed in the rotor. For example, Patent Document 1 discloses such a magnet assistance-type reluctance motor: a magnet is embedded in the rotor of the reluctance motor.
The magnet assistance-type reluctance motor is set in such a way that a difference in inductance between a direction of d-axis (central axis of a permanent magnet) and a direction of q-axis (which is electrically and magnetically perpendicular to d-axis) becomes large. In the rotor, reluctance torque Tr is generated. Since the permanent magnet is embedded in the rotor, magnet torque Tm is generated by the permanent magnet. Total toque Tt of the entire motor is: Tt=Tm+Tr. Therefore, the output torque is larger than the reluctance motor that only generates Tr. As a highly-efficient and high-toque motor, the magnet assistance-type reluctance motor has been widely used in recent years for electric power steering devices (which will be referred to as EPS when necessary), electric cars, hybrid cars, consumer electronics such as air conditioners, and driving sources such as those of various types of industrial machinery.
In the magnet assistance-type reluctance motor, total toque Tt is represented in the following manner. In general, so-called maximum torque control (advance angle control) is performed to maximize the torque generated for the same current.
                    Tt        =                ⁢                  Tm          +          Tr                                                  =                    ⁢                                                    p                ·                φ                            ⁢                                                          ⁢                              a                ·                Iq                                      +                          p              ·                              (                                  Ld                  -                  Lq                                )                                                    ⁣                  ·          Id          ·          Iq                    (p: number of pole pairs, φa: armature interlinkage magnetic flux by permanent magnet, Ld: d-axis inductance, Lq: q-axis inductance, Id: d-axis current, Iq: q-axis current)
During the maximum torque control, angle β between Id and Iq (current phase angle) is controlled in such a way as to generate torque in the most efficient manner with respect to armature current. The operation is conducted in a highly-efficient and high-torque manner.
However, in the magnet assistance-type reluctance motor, as the armature current increases, the ratios of magnet torque Tm and reluctance torque Tr to total torque Tt would change. Accordingly, the ratio of Tr tends to increase. In this case, since the current value is high, the effect of armature reaction becomes larger accordingly. As a result, the torque ripple becomes larger than when the current is low. In particular, after the reluctance toque exceeds 10%, the torque ripple rapidly increases. The problem is that, in the case of EPS, the torque ripple rate exceeds the upper limit or 5%.
In order to reduce the toque ripple in the magnet assistance-type reluctance motor, various methods have been proposed. For example, Patent Document 2 discloses a motor control device that calculates a torque ripple, calculates and supplies a current command value to generate an opposite-phase torque to the torque ripple in order to reduce the toque ripple. In this case, first, the toque ripple calculation means calculates a fundamental wave current in a dq coordinate system, and a toque ripple caused by higher harmonic wave components of armature interlinkage magnetic flux associated with a permanent magnet. Then, the torque ripple reduction higher harmonic wave current command value generator calculates a higher harmonic wave current command value to generate toque whose phase is opposite to that of the torque ripple calculated by the torque ripple calculation means. Then, the higher harmonic wave current control circuit controls higher harmonic wave current based on the higher harmonic wave current command value, thereby reducing the torque ripple of the motor.